In a connection-oriented communication network, the transfer of information between two end-users is accomplished by network functions that select and allocate network resources along an acceptable path. The logical association between the communicating end-users is referred to as a call. The chain of associated network resources that support the call forms a connection. Connection management is a network function that is responsible for setting up, maintaining, and taking down connections. Each call request is issued with a set of quality of service (QOS) requirements that govern the resource allocation for the desired connection. QOS requirements for a connection may be specified in terms of performance constraints (e.g., throughput, delay), resource constraints (e.g., carrier, security), and priority constraints (e.g., establishment priority, retention priority).
Conventional allocation of network resources for connection management is generally static. A fixed level of quality of service, specified by the user, is to be matched at connection setup for a call and must be maintained by best efforts throughout the duration of the call. Certain performance parameters, such as throughput (or rate), are negotiated to determine their respective agreed values. The same agreed values must ideally be met as long as the call is active.
Static allocation of network resources is inefficient, and is inadequate for a dynamic networking environment where the user requirements and the quality characteristics of network resources are not static. With user demands that vary with the time of the day, a network is prone to sporadic heavy loads. When network resources (e.g., links) are heavily utilized due to a transient increase of requests for connection establishment and/or re-establishment, time delays and network blocking increase. A network overload control strategy is needed to minimize such degradation in quality of service.
Dynamic routing may be used to steer connections away from saturated links. However, as routing paths meander around saturated links, they tend to be longer. In this respect, more network resources are used, resulting in even higher call blocking rates. A call carried on a long path can potentially block a significant number of other calls that might use the same resources for their connections. Another existing approach is to require incoming calls to wait for connection resources during periods of overload. Although blocking may be somewhat relieved, calls will generally experience a higher delay in connection establishment and re-establishment.
When a link fails, the network attempts to reestablish the affected connections. In a sparsely connected network, a small number of links must share the rerouted traffic. This could lead to an immediate saturation of those links. Many affected connections may thus fail to be reestablished. Those with a high re-establishment priority may still preempt existing connections with a lower retention priority. In any case, some calls would be sacrificed.
A time-of-day call is one that must acquire a connection during a predetermined period of time. In one approach of the prior art, network resources are reserved ahead of time for the time-of-day connection. In order to do that, the network must either add switched facilities when they are needed for the connection, or start accumulating resources for reservation in advance so that enough would be available with a high probability when the resources are needed. In another approach of the prior art, the time-of-day call is assigned a very high establishment priority so that it may acquire the needed resources by preempting existing calls with low retention priority.
Preemptive network resource allocation strategies rely on disruptive retrieval of network resources that have already been allocated to existing calls in order to accommodate new calls of greater importance. Non-preemptive alternatives, such as resource reservation, extended path search, limited waiting for resources, etc., have considerable drawbacks. For voice calls whose rates may be adjusted without affecting the continuing existence of their associated connections, an approach exists that involves the switching between DSI (digital speech interpolation) and non-DSI codings on a network-wide basis. Its control mechanism is strictly triggered by link failures. When there is a link failure, in some devices, every voice call in the network is subject to DSI provided it is permissible. When the failed link is subsequently recovered, the affected voice calls will regain their DSI-free coding.
A different method of dynamically controlling the rate of a connection is found in U.S. Pat. No. 5,115,429, "Dynamic Encoding Rate Control Minimizes Traffic Congestion in a Packet Network," by Michael G. Hluchyj and Nanying Yin. This method, known as dynamic rate control, uses a variable rate coder to adjust the source coding rate based on network feedback information. Temporary relief from internodal link congestion resulting in packet queue build-up, and improvement in statistical gain can be realized at the expense of temporarily degrading the quality of service for the connections that are subject to dynamic rate control. In this control scheme, congestion information is picked up and carried by a single bit in the protocol data units (fast packets or cells) and returned to the source by the destination. The source coder switches between a higher rate and a lower rate depending on the feedback information. If the feedback information indicates congestion, the switch is from the higher rate to the lower rate. If the feedback information indicates otherwise, the switch is the opposite. This technique is used for packet level congestion control and operates over time scales on the order of the round trip transmission delay (e.g., 100 msec), whereas the disclosed invention is used for connection-level overload control and operates over much longer time scales (e.g., minutes).
Thus, there is a need for a device and a method that manage call resource allocation on selected links in a connection-oriented communication network such that existing connections share the burden of releasing resources for accommodating new connections.